1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a device and method for printing an alignment layer and a mask for printing an alignment layer.
2. Background of the Related Art
In general, different types of flat panel displays are commonly implemented in various display apparatus, including Liquid Crystal Display (LCD), Plasma Display Panel (PDP), Electro Luminescent Display (ELD), and Vacuum Fluorescent Display (VFD). Of these different types, the LCD devices have commonly replaced apparatus that used Cathode Ray Tubes (CRTs) because of their excellent picture quality, light weight, thin profile, and low power consumption. In addition to mobile apparatus that use LCDs, such as monitors of notebook computers, the LCDs are increasingly being implemented for televisions and for monitors of home computers.
In general, an LCD device includes a liquid crystal display panel for displaying a picture, and a driving part for providing a driving signal to the liquid crystal display panel. The liquid crystal display panel includes first and second substrates bonded together with a gap formed between the first and second substrates, and a liquid crystal material is injected into the gap between the first and second glass substrates.
On the first substrate (commonly referred to as a TFT array substrate), there are a plurality of gate lines arranged along one direction at fixed intervals, a plurality of data lines arranged along a second direction perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes disposed within pixel regions defined by an intersection of the gate and data lines that form a matrix, and a plurality of thin film transistors switchable in response to a signal transmitted by the gate lines for conducting a signal from the data line to the pixel electrodes. On the second substrate (commonly referred to as a color filter substrate), there is a black matrix layer for shielding light from portions other than the pixel regions, a red (R), green (G), and blue (B) color filter layer for displaying colors, and a common electrode for implementing a picture.
The first and second substrates are spaced apart by spacers, and bonded together by a sealant material. The sealant material includes a liquid crystal material injection hole, through which the liquid crystal material is injected. Physical characteristics of the liquid crystal material are dependent on molecular arrangement of the liquid crystal molecules, and may be altered by application of an external force, such as electric field. Accordingly, filling of the liquid crystal material between the first and the second substrates cannot provide uniform molecular arrangement required for proper operation of the LCD device. Thus, an alignment layer is formed upon a surface of each of the first and second substrates.
In general, main composition materials for forming the alignment layers commonly include inorganic or organic substances. Of these main composition materials, polyimide group materials are generally considered better as compared to other organic polymers with respect to printing, rubbing, alignment control performance, and chemical stability. Currently, the polyimide group materials are commonly employed as a material for forming alignment layers of various LCD devices.
During formation of the alignment layers, diamine and acid anhydride are made to react in a solvent to prepare formation of polyamic acid. The material used during printing is the polyamic acid, whereby the polyimide is obtained as the polyamic acid is dried and set by application of heating. The polyimide alignment layer may be formed by various processes including spinning, spraying, dipping, and printing.
FIG. 1 is a schematic view of a device for printing an alignment layer according to the related art. In FIG. 1, the device includes a raw material tank 103 having raw material 101, a raw material supply tube 104, a dispenser 100, an anilox roll 120, a doctor roll 110, and a printing roll 130.
A mask 210 is positioned on the printing roll 130, and is formed of a printing rubber plate with a 30% numerical aperture. The numerical aperture is defined as a ratio of a portion of mask that does not have the raw material 101 to a portion of the mask that has the raw material 101. Generally, a mask 210 with a numerical aperture below 30% is employed for an LCD device having a resolution class below a high resolution XGA (1024×768 class).
In order to flow the raw material 101 through the raw material supply tube 104, nitrogen gas (N2) is injected into the raw material tank 103. When the nitrogen gas (N2) is supplied to the raw material tank 103, the raw material 101 is dropped from the dispenser onto the rotating doctor roll 110 and the anilox roll 120 via the raw material supply tube 104. The raw material 101 supplied to the doctor roll 110 and the anilox roll 120 is kneaded between the doctor roll 110 and the anilox roll 120, whereby the raw material 101 is evenly coated onto the surface of the anilox roll 120. Then, the evenly coated raw material 101 on the anilox roll 120 is transferred onto the substrate 150 that is positioned on the printing table 160 by the printing roll 130. Accordingly, the masks 210 positioned on the printing roll 130 each have a 30% numerical aperture such that the substrate includes portions having the raw material 101 and portions not having the raw material 101. Finally, the raw material 101 positioned on the substrate 150 is cured, thereby forming the alignment layer.
FIGS. 2A–2C are plan and perspective views of a mask for printing an alignment layer according to the related art. In FIG. 2A, a matrix of masks 210 having a plurality of projections 220 are positioned on a substrate 200, wherein each of the masks 210 is formed of printing rubber plate.
In FIG. 2B, during transfer of the raw material 101 from the printing roll 130 onto the substrate 150 (in FIG. 1), no raw material 101 is transferred from regions having the projections 220. Accordingly, the raw material 101 cannot be transferred to the substrate 150 (in FIG. 1) where the projections 220 contact the substrate 150. If a mask 210 without the projections 220 is used, the raw material 101 cannot be uniformly coated onto the surface of the mask 210 uniformly, thereby forming blots of raw material onto the substrate 150 (in FIG. 1). Thus, a plurality of openings 220 are formed in the surface of the mask 210 for uniform transfer of the raw material 101 onto the substrate 150 (in FIG. 1). In addition, defective printing of the raw material 101 onto the substrate 150 is proportional to an area of the substrate 150 having no raw material 101 printed thereon. Moreover, LCD devices classified below the high resolution class that have large sized pixels also have a lower ratio of defect occurrence caused by infiltration of contaminants than LCD devices classified above the high resolution class even using the mask 210 having a 30% numerical aperture.
In FIG. 2C, the raw material 101 is transferred onto the substrate 150 (in FIG. 1) except where regions correspond to the projections 220 on the mask 210. Accordingly, the 30% numerical aperture mask 210 is problematic when implemented for fabricating LCD devices classified in the high resolution class or higher having small unit pixels. Since the 30% numerical aperture mask 210 includes the projections 220, contaminates, such as dirt, are transferred onto the printing roll and onto the substrate.